The present invention relates to an idle revolution control device for an internal combustion engine, in which the revolution of the engine at an idling condition is controlled to a predetermined value according to a temperature of an engine coolant.
A typical example of such idle revolution control device is shown in FIG. 1, in which reference numerals 1, 2, 3, 4, 5, 6 and 8 depict an internal combustion engine, a thermister as a water temperature sensor for detecting a coolant temperature of the engine 1 and providing an electric signal representative of the temperature, an engine revolution sensor for detecting the number of revolutions of the engine 1, a throttle valve provided in an intake pipe for controlling an amount of intake air, an idle switch for detecting a full closure of the throttle valve 4, i.e., an idling condition, a control device including a CPU 7 and a water temperature sensor interface 9 and an actuator provided in a bypass conduit bypassing the throttle valve 4, respectively.
The CPU 7 of the control device 6 receives outputs from the water temperature sensor 2, the revolution sensor 3 and the idle switch 5 to drive the actuator 8 according to these informations to cause it to regulate air flow through the bypass conduit to thereby control the idle revolution of the engine 1. The water temperature sensor interface 9 comprises a voltage dividing resistor R.sub.1 for converting an output resistance of the thermister 2 into an analog voltage and a series connection of a resistor R.sub.2 and a capacitor C.sub.1 which constitute a primary filter for noise removal. An input voltage from the thermister 2 to the control device 6, an input voltage to the CPU 7 and a source voltage are depicted by V.sub.1, V.sub.2 and V.sub.3, respectively.
In operation, an engine water temperature information from the thermister 2 and the output of the idle switch 5 are supplied to the CPU 7. When the CPU 7 confirms, according to the signal from the idle switch 5, that the engine 1 is idling, it calculates a desired revolution number of the engine on the basis of the information from the thermister 2 and a known relation between the information and the desired revolution number which is shown in FIG. 3, compares the desired revolution with an actual revolution number detected by the revolution sensor 3 and provides a drive signal which is supplied to the actuator 8. The actuator 8 responds to the drive signal to regulate the amount of air flowing through the bypass conduit so that a difference between the calculated value and the actual value is minimized. Thus the idling revolution is controlled. The controlled idling revolution is detected again by the revolution sensor 3 and by repeating this operation, the idling revolution number is finally controlled to a predetermined value.
It has been known practially, however, that there is a tendency of temporal disconnection or intermittent disconnection, i.e., chattering, between the thermister and the control device 6 due to undesired vibratioan or shocks of a vehicle equipped with them. FIGS. 2A to 2C illustrate voltage waveforms at various portions of the control device when such temporal disconnection and chattering between the thermister 2 and the control device 6. When the thermister 2 is completely disconnected from the control device 6 as shown in FIG. 2A, the input voltage V.sub.1 to the control device 6 abruptly rises from a thermister output voltage V.sub.4 to the battery voltage V.sub.3. At this time, the resister R.sub.1 serves as a pull-up resister which also serves to fix the voltage at the disconnection. Further, the input voltage V.sub.2 to the CPU 7 rises also gradually to the battery voltage V.sub.3 with a rising rate being determined by the time constant of the R.sub.2 C.sub.1 circuit and, when the input voltage V.sub.2 to the CPU 7 becomes higher than a predetermined value set to discriminate the disconnection, the CPU 7 controls the fuel injection regardless of the information from the water temperature sensor to an extent that a reckless operation of the engine is restricted.
When the temperature sensor 2 is disconnected temporarily from the control device 6 as shown in FIG. 2B, the input voltage V.sub.1 to the control device 6 rises abruptly from V.sub.4 to V.sub.3 and then falls to V.sub.4. The input voltage V.sub.2 to the CPU 7 rises toward V.sub.3 and, when the disconnection is terminated, starts to fall to V.sub.4 with a falling rate being determined by the time constant C.sub.1 R.sub.2 which is usually several milliseconds.
Considering the fuel economy, it is ideal that the idling revolution of the engine is minimum at which the engine can rotate smoothly at a given temperature. Therefore, it has been usual that the desired revolution decreases with increase of the coolant temperature, as shown in FIG. 3. Further, since the resistance of the thermister 2 decreases with increase of temperature, both the input voltages V.sub.1 and V.sub.2 are low at high temperature and high at low temperature. Therefore, when a normal output voltage of the thermister 2 is V.sub.4, the input voltage V.sub.2 changes from V.sub.4 through V.sub.5, V.sub.3 and V.sub.5 to V.sub.4.
For this reason, the desired revolution which should be N.sub.1 becomes N.sub.2 corresponding to V.sub.5, which is too high.
In the case of the chattering as shown in FIG. 2C, the input voltage V.sub.2 vibrates between the normal voltage V.sub.4 and the battery voltage V.sub.3. Assuming 50% duty cycle chattering, the input voltage V.sub.2 may be astringent to an intermedial value between V.sub.4 and V.sub.3. Therefore, by changing the duty cycle suitably, it is possible to set the input voltage V.sub.2 to an arbitary value between V.sub.3 and V.sub.4 and so the desired revolution of the engine 1 is selected in a range from N.sub.1 to an upper limit of control.
As mentioned, various signals corresponding to abnormal conditions which do not correspond to water temperature are sent to the CPU 7 when the thermister 2 is disconnected temporarily or intermittently from the control device 6 and, when the CPU 7 responds to all of such signals, a range of the desired revolution of the engine becomes wide enough to cover all control range and, for some extreme case, the engine revolution rises abnormally.